Numerous types of medical equipment, as is well known, are used to reconstruct an image of the interior of a subject across a plane. Examples of such medical equipment include x-ray tomographic scanners, magnetic resonance imaging scanners, and nuclear medicine emission computed tomography (ECT) scanners. Each of these scanners are used to reconstruct an image of a cross section, or slice, through a patient's body. Consecutive cross sectional images are then combined to create an image of the interior of a subject.
Tomography is a medical technique of radiographic analysis which provides an image of a particular plane of a body under examination. In one form of tomographic instrument known as a CAT scanner (computed axial tomography), an x-ray source and x-ray detectors are positioned in alignment on opposite sides of a subject under examination and simultaneously scan an edge of a cross sectional plane or slice of finite thickness extending through the subject.
Intensity of x-ray transmission through the subject is determined by sampling an electrical output of the detectors. Sampling occurs in numerous locations in the direction of a single scan. The x-ray source is then rotated a predetermined angular distance about an axis normal to the plane or slice through the subject and another scanning of the edge in a different direction is obtained. Resultant data is processed by a computer to reconstruct an image of the planer cross-section or slice through the patient's body.
In another form of image reconstruction known as magnetic resonance imaging (MRI), the body of a subject is placed within a magnetic field. When a radio-frequency at the resonance (Larmor) frequency is applied to the subject within the magnetic field, the magnetic moment of the subject's atoms which are normally in random alignment align in a north and south direction relative to the magnetic field. When the radiofrequency is terminated, the atoms return to their random alignment and in so returning will emit energy via a radiofrequency at the same resonance frequency. This radiofrequency is detected via an antenna and the resultant data generated by the detection can be analyzed into contributing frequencies and processed by a computer to reconstruct an image in cross section of the patient's body. This form of image reconstruction is also known as nuclear magnetic resonance (NMR) imaging.
Image reconstructing apparatuses include such CAT scanners, MRI or NMR scanners, ECT scanners, and any apparatus that receives data and processes it to create an image in cross section of the patient's body.
It is desirable at times to verify that the operation of an instrument for reconstructing an image of the interior of a subject conforms with its known capabilities. In addition, it is desirable to predetermine the capabilities of the instrument for the performance of specific examinations.
A test body (phantom) for determining the operating characteristics of a scanning tomographic analytical apparatus of the type known as a CAT scanner is disclosed in co-assigned U.S. Pat. No. 4,055,771, issued Oct. 25, 1977 to Goodenough et al. The contents of this U.S. Patent in its entirety are hereby incorporated into this application in order to more fully define the state of the art to which the subject invention pertains. The test body comprises energy absorption means arranged in layered arrays extending generally parallel to a direction of projection of the x-ray beam, and means for positioning the energy absorbing means between a scanning x-ray beam and a transmission intensity detector of the tomographic apparatus.
A phantom for determining the operating characteristics of a nuclear magnetic resonance scanner is disclosed in U.S. Pat. No. 4,644,276, issued Feb. 17, 1987 to Newman and Sierocuk. The phantom comprises at least two test plates which include means for testing parameters of the scanner and allows the scanner to be tested in the plane in which these test plates lie.
Another phantom for determining the operating characteristics of a magnetic resonance imaging scanner is disclosed in U.S. Pat. No. 4,692,704, issued Sep. 8, 1987 to Gray. The phantom includes a generally tubular body containing a cylindrical stack of a plurality of leaves. Each leaf has one or more wedge shaped slices or sectors cut out for identification and reference purposes.
These test bodies, or phantoms, used for determining the operating characteristics of an image reconstructing apparatus often include at least one test element adapted for testing the resolution of the apparatus. One such test element comprises a resolution gauge having line pairs of predetermined width. These resolution gauges have been cut from plastic of varying densities and some metals. The technologies used to cut plastic resolution gauges were pushed to the limit of standard machining tolerances to produce accurate resolution of eleven line pair per centimeter (line pair). As the technology of image reconstructing apparatuses has improved and progressed, the high resolution of such apparatuses has increased to greater levels. The new image reconstructing apparatuses require high resolution running up to twenty line pair per centimeter. To achieve the tolerances necessary to create accurate tests for resolution above eleven line pair, different technologies of cutting materials were required.
The subject invention has solved this problem by providing a test element adapted for testing the resolution of the apparatus produced by using a wire electrical discharge machine in order to cut line pairs of predetermined width into metal. Using this technology, line pairs up to twenty line pairs per centimeter are achieved. Additionally, plastic resolution gauges have been obtained by using laser technology to cut line pairs in plastic. These plastic resolution gauges are able to be cut up to eleven to thirteen line pairs per centimeter. Use of the wire electrical discharge machine and the laser technology have provided resolution gauges which solve the problems required by the high resolution obtainable with image reconstruction apparatuses.
Test bodies for determining the operating characteristics of image reconstruction apparatuses also often include at least one test element adapted for testing the modulation transfer function of the apparatus. The test element, as disclosed in co-assigned U.S. Pat. No. 4,055,771, previously comprised a cylindrical wire which is used to create a small point in a scanned image which will generate the information required to calculate the modulation transfer functions. As the resolution of image reconstructing apparatuses increased dramatically, dimensionally smaller and more symmetrical point sources were required. The wire type point source is positioned perpendicular to the scan plane, creating a small cross sectional footprint of the scan image. However, if the wire is not maintained in a perpendicular position to the z axis, the image created when the wire is scanned through the xy plane will be oval instead of circular, and often leads to streak artifacts. This elongation will reduce the accuracy of the MTF calculation.
The subject invention provides a bead which is used as the impulse point source for modulation transfer function. The bead is three dimensionally symmetrical which eliminates the z axis alignment positioning dependency which is required with the cylindrical wire. The bead size and shape eliminate the requirements for exact perpendicular alignment to the z axis, reducing the requirement for accurate alignment. This is especially helpful in daily quality assurance checks when the technician is rushed and unable to take the time for careful alignment, and when the image evaluation is being performed by a computer and the technician may not be aware of the critical nature of proper positioning. The subject invention thus solves the problems associated with previous cylindrical wires used as point sources for modulation transfer function.